As disclosed in a patent document 1 (i.e., Japanese Patent Laid-Open No. JP-A-2004-324589), a vehicle having an idle-stop function may have an over-revolution prevention function to prevent an over or excessive revolution speed of the engine by applying a load torque from the motor-generator to the engine when the engine is re-started. In such configuration, the load torque from the motor-generator is proportionally increased when the engine revolution speed becomes high.
In the technique of the patent document 1, the load torque from the motor-generator is increased in proportion to the level of the engine revolution speed at the engine start time. However, when the load torque from the motor-generator is steeply increased, such a steep load torque increase may cause vibration of the vehicle. A steep increase may be a sharp, immediate, sudden, or abrupt torque increase or torque spike.
Further, in the technique of the above-mentioned patent document 1, since the load torque from the motor-generator is increased only when an accelerator opening is fully closed at the engine start time for the prevention of the over-revolution of the engine, the over-revolution of the engine at a certain engine start time, such as when the accelerator opening is not fully closed or when the accelerator is open even in a little degree, will not be prevented. More practically, when the engine in the hybrid vehicle is started in response to an acceleration request during a travel of the vehicle, excessive engine speed may not be prevented by the technique in the patent document 1.